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CX2SA > GPS 01.07.06 18:46l 82 Lines 4220 Bytes #999 (0) @ WW
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Subj: GPS can help give early...
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From: CX2SA@CX2SA.LAV.URY.SA
To : GPS@WW
GPS can help give early warning of tsunamis
===========================================
Using GPS (global positioning system) data to measure how points on land move
following an undersea earthquake could help geologists decide if the tremor
will cause an ocean-wide tsunami. Combined with existing tsunami warning
systems, the data could speed up decisions about whether to issue an alert and
avoid false alarms, say US researchers.
Tsunami warning systems use a combination of seismometers to measure tremors
and ocean buoys to spot pressure waves. But it is difficult to pinpoint the
exact strength of an undersea quake. This can cause problems for those
deciding whether to issue a warning, says Seth Stein, a geophysicist at
Northwestern University in Illinois, US.
"The hardest job is to distinguish quakes that are big from those that are
dangerously big," he told New Scientist. "Richter scale 8 is quite a big
earthquake, but about 8.5 is the magic number. Above that, ocean-wide tsunamis
start to happen."
GPS measurements of points around a quake could determine more quickly than
current methods whether this threshold has been exceeded, he says.
Vague measurement
-----------------
The 2004 Sumatran quake - which caused the Asian tsunami - was eventually
measured at between 9.2 and 9.3, but seismometers can initially only determine
whether a quake is larger than about 7.
"It normally takes a couple of hours to know whether it was over 8.5 or not,"
says Stein. "Limits on how much energy can be stored in rock mean the first
body waves of a quake don't get bigger, but just ring for longer."
Taking GPS measurements of points on land around a quake can answer this
crucial question within 15 minutes, according to a study by Stein and co
-workers from the University of Nevada, US.
To prove the technique can work, they used GPS data recorded during the first
15 minutes of the 2004 Sumatran quake, which made it clear the tremor would go
on to cause a devastating tsunami in the Indian Ocean.
Millimetre accuracy
-------------------
Software developed at NASA's Jet Propulsion Laboratory was used to measure the
position of 38 GPS stations between 300 and 7500 kilometres from the quake's
epicentre northwest of Sumatra. Knowing how these stations moved to within 7
millimetres makes it possible to measure long-frequency waves from the quake
and estimate its size.
"Using that 15 minutes of data it was possible to say the quake was a 9." This
was very close to the 9.2 or 9.3 figure eventually determined for the quake,
says Stein. "We think this would be a useful third component to the existing
tsunami warning system. By taking out the guesswork it could make it more
accurate and avoid false alarms."
Geophysicist Paul Burton at the University of East Anglia, UK, agrees, but
cautions that the system's effectiveness will depend on the location of the
earthquake. "It's feasible but might not help in all circumstances."
If the GPS stations available are not located in the right place relative to
the epicentre, satellite measurements may not be that helpful, he says.
"Another consideration is whether high-tech systems for tsunami warnings will
be here in three to four hundred years time," says Burton. "Educating people
right now and making sure the knowledge about what to do is kept alive can
make a huge difference. For example, a 10-year-old British girl saved many
lives during the Indian Ocean tsunami when she remembered the early signs of a
tsunami from her geography lessons."
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